1. Field of the Invention
The present invention relates to polymeric coatings on shape memory materials. More specifically, it relates to polyimide coatings on shape memory alloys that are capable of withstanding elevated temperatures required to effectively form shape memory materials into a desired configuration.
2. Description of the Related Art
Shape memory materials, such as shape memory alloys, include materials having anthropomorphic qualities of memory and trainability. A particularly useful attribute of these alloys is that when they are plastically deformed at a particular temperature, they can completely recover their previous shape by superelastic or shape memory mechanisms after receiving the appropriate thermal mechanical conditioning procedure. The defined shape or size can be achieved through thermomechanically shapesetting the materials. For example, the shape memory material is typically processed by being restrained in the desired memory shape while increasing the temperature within the material to set this shape.
The ability to revert to a previously defined shape or size has broad applicability in many different fields. For example, the medical industry has incorporated shape memory materials into a number of products including catheter guidewires, stents, surgical hooks, vasoocclusive devices, suturing and stapling devices. Many other consumer and industrial products have been fabricated from the shape memory materials, including eyeglass frames, cellular telephone antennas, damping devices, orthodontic arches, brassiere underwires and the like.
In many of the above-identified applications, it is desirable or necessary to apply a polymeric coating to the surface of a component fabricated from shape memory material. For instance, a coating may be required to extend the component""s useful life, act as an adhering surface for bonding additional polymeric features, act as an insulator, enhance biocompatibility of the component and the like.
Heretofore, if a polymeric coating was required on a component made of shape memory material, the coating was applied after thermomechanical heat treatment to shape-set the material because the polymeric materials utilized for coating were incapable of withstanding the elevated temperatures required for subsequent thermomechanical shape-setting the material. However, applying a polymeric coating, after the shape-setting process, is labor-intensive, requiring multiple steps and additional time. Furthermore, a number of quality control factors are presented that impact the final product, such as lack of uniformity in the coating thickness and inability to apply very thin films to the surface.
Heretofore, coatings have been applied by processes that involved several steps. For example, U.S. Pat. No. 5,443,907 discloses a method for coating medical insertion guides. The method described by the ""907 patent includes installing a polyurethane sleeve on a core wire, which may be a preshaped and heated nitinol wire, and heating the polymeric sleeve to a temperature until it is reformed around the core. However, such a method, entailing several manual steps, increases the cost of production and decreases efficiency.
Accordingly, there is a need for improved polymeric-coated shape memory materials and methods of making same that provide precoated shape memory elements that withstand the elevated temperatures required during shape-setting treatment, have coatings with essentially uniform thickness, have very thin coatings if desired, and minimize time and effort required for the coating process thereby increasing efficiency and decreasing cost.
As used herein, the terms and expressions below, appearing in the specification and claims, are intended to have the following meanings:
xe2x80x9cShape memory propertiesxe2x80x9d as used herein includes thermal shape memory effects characterized by the ability of a material to recover a pre-set shape upon heating; and mechanical superelasticity effects characterized by the ability to withstand high elastic strain and recover the initial shape after release of the strain.
xe2x80x9cThermomechanical treatmentxe2x80x9d as used herein in reference to treatment of a shape memory material means a heat treating regime to shape-set the shape memory material where the material may or may not be restrained during the treatment.
A principal object of the present invention is to provide a shape memory material coated with a polymeric coating wherein the polymeric coating is applied before thermomechanical treatment of the shape memory material to activate the shape memory properties and shape-set into the desired configuration.
Another object of the present invention is to provide a polyimide coating that withstands higher temperatures required to shape-set the shape memory material.
A further object of the present invention is to provide an improved method of coating shape memory material to decrease overall production costs and increase efficiency.
A still further object of the present invention is to provide a method for increasing adhesion of polymeric materials to shape memory materials thereby overcoming problems associated with non-adhering polymer extensions.
Another object of the present invention is to provide essentially uniform coatings on surfaces fabricated from shape memory material.
Yet another object of this invention is to provide a coated shape memory material that exhibits reduced temperature variations in the shape memory material when the shape memory element is heated electrically by I2R and held in contact with shaping guides and fixtures which act as a heat sink.
Still another object of this invention is to provide electrically and thermally activated devices fabricated from the disclosed polyimide coated shape memory material having electrical insulating abilities for high dielectric isolation from other components in the systems.
These and further objectives are accomplished by the materials and methods disclosed herein.
In one aspect, the present invention relates to a shape memory material coated with at least one cured polymeric material that is capable of withstanding the elevated temperatures normally required to effectively thermomechanically shape-set the shape memory material into a desired configuration. Advantageously, the polymeric material may be cured previously to or concurrently with the thermomechanical shape-setting process.
Generally, any polymeric material, that in the cured stated exhibits heat resistant properties to withstand the elevated temperatures of the shape-setting process, may be employed. Illustrative polymeric materials having utility in various applications of the invention include, without limitation polyimides, polybenzimidazoles, polybenzoxazoles, polybenzothiazoles, polyoxadiazoles, polytiazoles, polyquinoxalines, polyimidazopyrrolones, and bismaleimide, polyamideimide, and compatible mixtures, blends and copolymers (of monomers) thereof.
In one preferred embodiment of the present invention, a cured polyimide coated element of a shape memory alloy is provided by coating an element of the shape memory alloy, which has not been thermomechanically heated to exhibit shape memory properties, with a polyimide material on at least a portion of the element of the shape memory alloy. The polyimide coating may be cured and then subjected to a thermomechanical heating regime necessary to exhibit shape memory properties. In the alternative, the curing of the polyimide coating may be simultaneously achieved during the thermomechanical heating regime. The polyimide coating is heat cured at a sufficient temperature to withstand the elevated temperatures of the subsequent or concurrent thermomechanically heating regimes required to shape-set the desired configuration.
Serendipitously and unexpectedly, it has been found that if the polyimide coating is heat cured within a certain elevated temperature range, the softening temperature and melting temperature of the polyimide is driven higher thereby allowing the polyimide coating to withstands higher temperatures. As a result, the polyimide coating is able to maintain its stability by retaining polymeric properties and behavior during the shape-setting heat treatment and after. Generally, the heat curing temperature range is from about 200xc2x0 C. to about 400xc2x0 C., preferably from about 250xc2x0 C. to about 350xc2x0 C., and more preferably from about 300xc2x0 C. to about 350xc2x0 C. Curing time may be dependent on the thickness of the coating which is predetermined by the specific application of the component. Generally for a thin film of from about 1 to about 2 mils, the curing time will range from about 1 minute to about 5 minutes.
The cured polyimide coated shape memory elements preferably are characterized by having essentially uniform coatings, at a desired thickness, on all coated surfaces to provide advantages including uniform heating through the entire element of component during the thermomechanical heating regime for shape-setting.
Other objects, features and embodiments of the present invention will be more fully apparent from the ensuing disclosure and appended claims.